1. Field of the Invention
The present invention relates to an acoustic surface wave element, which can be used for a voltage controlled oscillator (VCO), a resonator, a filter, and like components of communication equipment, such as a car telephone and a cordless telephone, and of audio equipment, such as a video tape recorder (VTR). In the above equipment, an acoustic surface wave element is currently widely used at a frequency of 10 MHz to 1 GHz, and must have a wider frequency deviation range and an excellent stability characteristic relative to temperature when used, for example, as a VCO.
2. Description of the Related Art
An acoustic surface wave delay line is known which comprises a substrate of a propagation media and a surface layer of a media having a temperature dependency delay time opposite to that of the substrate so that the temperature dependency delay time of the substrate is reduced (Japanese Unexamined Patent Publication (Kokai) No. 47-37154, published on Nov. 30, 1972). This disclosure relates to a general reduction of the temperature dependency delay time of a propagation media and is exemplified only by a combination of a fused quartz aluminum. The propagation mediums mentioned are lithium niobate, lithium tantalate, and cadmium surfide.
A silicon dioxide layer for the above surface layer has been proposed, to reduce the temperature dependency delay time of propagation mediums of an acoustic surface wave device (U.S. Pat. No. 3,965,444, issued on June 22, 1976). As piezoelectric materials used for the propagation mediums, there are mentioned YZ-cut lithium niobate and YZ-cut lithium tantalate (col. 4, 1.2-4 of USP' 444), but such an acoustic surface wave device is difficult to manufacture because, in the case of the YZ-cut lithium tantalate (the acoustic velocity therein is 3230 m/s), at 150 MHz the hK (h=film thickness of the silicon dioxide layer and K=2.pi./.lambda. where .lambda.=acoustic wavelength) becomes 3 (col. 4, 1.31-35 of USP' 444), and thus the thickness of the silicon dioxide layer must be at least 10.3 .mu.m. A silicon dioxide layer having a thickness of 10 .mu.m causes an increased stress in the layer and a longer deposition time (for example, about 11 hours at a deposition rate of 150 angstroms/min), among other disadvantages.
Also known is an acoustic surface wave element comprising a combination of SiO.sub.2 /LiTaO.sub.3 (Japanese Unexamined Patent Publication (Kokai) No. 55-159612, published on Dec. 11, 1980). This invention uses an X-cut LiTaO.sub.3 at an acoustic propagation direction of 112.degree. to the Y-axis. This acoustic surface wave element obtained a large electromechanical coupling factor of 1.44%, but this is a small electromechanical coupling factor for an acoustic surface wave element using a single crystal of LiTaO.sub.3 and allows only a narrow frequency deviation range for the acoustic surface wave element. Further, the electromechanical coupling factor is varied depending on the thickness of the SiO.sub.2 layer, and thus it is difficult to manufacture this type of an acoustic surface wave element. Furthermore, a problem arises in that, with an increase of the thickness of the SiO.sub.2 layer, the inductance component is reduced and the equivalent series resistance is increased, finally resulting in a stopping of the oscillation.
Further, there is reported a structure of SiO.sub.2 /36.degree. Y-X LiTaO.sub.3 for an acoustic surface wave device (W. Chujo et al, "SiO.sub.2 /LiTaO.sub.3, LiNbO.sub.3 Structure Acoustic Surface Wave Materials Fabricated by Plasma CVD Method", Dentsu Gakkai Cho-ompa Kenkyu Shiryo US 80-3, Apr. 1980, pp. 15-20; and W. Chujo et al, "SiO.sub.2 /LiTaO.sub.3, LiNbO.sub.3 Structure Acoustic Surface Wave Materials Fabricated by Plasma CVD Method", Dentsu Gakkai Cho-ompa Kenkyu Shiryo US 79-16, Jun. 1978, pp 25-30). Here, a zero temperature coefficient of a delay (TCD) is reported at T/.lambda.=0.11 (T is the film thickness of the silicon dioxide layer and .lambda. is a wavelength). From experiments by the present inventors, it was found that a relatively thick aluminum layer is necessary to attain a zero TCD at a T/.lambda. (where T is a thickness of the silicon dioxide layer) of about 0.11, and the t/.lambda. (where t is the thickness of an aluminum layer) becomes more than 0.05, lowering the oscillation characteristics.
Quartz can be used as a substrate of an acoustic surface wave element to obtain not more than 100 ppm of TCD at -20.degree. C. to +70.degree. C., but quartz has a very small electromechanical coupling factor and is not suitable for use in a VCO with a wide frequency deviation range.